since Vit-C is difficult or not available, or only very expensive or in poor quality in some parts of the word, I thought it might be useful to post here a wordwide supplier of affordable 100 % pure ascorbic acid (Vit-C). It's a family owned business in the 4th generation, located in Germany and usually supplies bakeries and grain mills with bigger quantities. Smaller quantities he sells via an ebay.co.uk shop worldwide. Mr. Breinbauer's answer on my email inquiry was very fast and friendly. Prices and worldwide shipping are very attractive, no additional taxes.
You should store the Vit-C in the dark and in airtight boxes or bottles.
Added to the link list. I'm not related in any way to the owner of the shop, I just post the link for your conveniance.